The present invention relates to a method for making a commutator which is capable of properly contacting with a brush and preventing any noise production by heating a molded product and by cutting protruding portions caused due to thermal expansion.
Generally, a commutator is an electric device comprising at least a pair of commutator members of a conductive material, and a commutator body of an insulating material. The commutator body supports the commutator members so that the commutator members can be properly spaced apart from each other and insulated. Thus, electric current inputted through an input brush can be transmitted to an output brush through the commutator members, and in such a situation, Direct Current (hereinafter referred to as DC) is inverted into Alternating Current (hereinafter referred to as AC), and vice versa.
Such a commutator is made by the method illustrated in FIG. 1, and will be briefly described below.
First, commutator members 132 are made of conductive materials such as copper, etc. The commutator members 132 are formed by drawing, or rolling process. Then, a plurality of such formed commutator members 132 are arranged around an inner circumferential surface of a metal mold 102 in a radial manner.
Then, an insulated resin such as phenol resin is injected or extruded into an inner space of the metal mold 102. Such injected or extruded insulating resin forms a commutator body 135.
Then, an unfinished product is separated from the metal mold 102 and shaped by a lathe to become a complete real circular-cross sectioned product.
The conventional method for making the commutator, however, has the following drawback.
While the commutator is rotated, heat is produced by the friction between the outer circumferential surface of the commutator and the input and output brushes, and also by the electrical resistance of the commutator members 132. Accordingly, deformities appear on the surface of the commutator.
It has been proved that the temperature of the commutator rises approximately to sixty to eighty degrees Centigrade (60xc2x0 C.-80xc2x0 C.) by the friction, or the electrical resistance when the commutator is rotated. Accordingly, the commutator body 135 and the commutator members 132 experience thermal expansion. The problem that arises is that the coefficients of thermal expansion of the commutator body 135 and the commutator members 132 are not same. That is, the commutator body 135 made of the resin material has greater coefficients of thermal expansion than those of the commutator members 132 made of metal. Accordingly, as shown in FIG. 2, the commutator body 135 expands outward between the commutator members 132, and form protruding portions having a predetermined length (d). Here, the length (d) of the protruding portions of the commutator body 135 are a few micrometers (xcexcm) so that it is hardly noticeable to the naked eye. In FIG. 2. however, the protruding portions are overemphasized for better understanding.
Thus, as some portions of the outer circumferential surface of the commutator body 135 protrude, the commutator body 135 can not make proper contact with the input and output brushes 122 and 124, and performance reliability is deteriorated, accordingly. Further, considerable noise is produced between the commutator and the input and output brushes 122 and 124.
An object of the present invention is to provide a method for making a commutator whose body does not protrude between the commutator members while the commutator body and the commutator members experience thermal expansion during the rotation of the commutator.
The above object will be accomplished by a method for making a commutator according to the present invention comprising the steps of arranging at least one pair of commutator members inside a metal mold in such a manner that the commutator members are spaced apart from each other, molding a product by injecting an insulating resin into the mold; shaping the molded product to the product having a real circular sectional cross section; heating the product to a predetermined temperature; and cutting the protruding portions of the commutator body caused by thermal expansion so as to align the outer circumference of the commutator body with the surface of the commutator members. Here, the predetermined temperature approximately ranges from sixty to eighty degrees Centigrade(60xc2x0 C.-80xc2x0 C.).
According to the commutator of the present invention, grooves are defined at the commutator body between the commutator members. Due to the presence of the grooves, the sectional cross section of the commutator can form a complete spherical shape when the commutator body undergoes thermal expansion due to the temperature rising while the commutator is rotated. Accordingly, the commutator and the input and output brushes properly contact each other, and noise does not occur.